1. Field of the Invention
This invention relates to a pneumatic tire comprising a tread provided on its land portion or block with a plurality of sipes each cut into a radial direction of the tire and extending substantially in a widthwise direction of the tread and developing excellent wet performances, performances on the ice and the like. This invention also relates to a vulcanization mold used for manufacturing the tire.
2. Description of Related Art
For example, if the sipe is formed in the block, the rigidity of the block having the sipe is necessarily lowered as compared with that of the block having no sipe, whereby it is apt to cause uneven wear such as heel and toe wear or the like. In order to control of the occurrence of such an uneven wear, for example, JP-A-5-58118 has proposed a technique wherein a convex portion is formed in a wall face of the sipe formed in the block to extend in the widthwise direction of the tire and a dimple engaging with such a convex portion is formed in the other wall face of the sipe. This publication discloses that according to this technique, sub-block portions of the block divided by the sipe control the falling with each other based on the engagement between the convex portion and the dimple in the application of braking force to the tire and hence the fallen quantity can be decreased to shorten the braking distance in case of braking on a frozen road surface and control the occurrence of heel and toe wear on a dry road surface.
In such a conventional technique, however, the sizes of the convex portion and the dimple to be formed on the sipe wall faces are small. Hence it is difficult to always produce a reliable engagement of both and hence there is highly caused a fear of providing no given effect. Additionally, when the braking force is applied to the tire, as shown in FIG. 5, the convex portion p of the sipe wall face is engaged with the dimple d of the opposite sipe wall face to restrain the fallings of the sub-blocks sb to each other as shown in FIG. 5a. On the other hand, when the traction force is applied to the tire, the engagement between the convex portion and the dimple is loosened to decrease the restraining force against the falling of the sub-block sb as shown in FIG. 5b. In the conventional tire, therefore, there is a problem that uneven wear is unavoidably caused in the application of the traction force.